1. Field of the Invention
This invention relates generally to a process for forming pressure vessels of an improved design and, more particularly, to a process for forming a pressure vessel by drawing and ironing to provide selectively controlled side-wall thicknesses, affording a structure of reduced material consumption and thus reduced weight while maintaining the required strength of the pressure vessel for use in its intended purpose.
2. Description of the Prior Art
Conventional pressure vessels, such as those used in the filter industry, are typically "drawn" vessels or containers. Drawing is a known metal fabrication technique wherein a given blank of material, particularly metal as used in pressure vessels, is reformed or reshaped to a particular, desired configuration. Drawing effectively retains or maintains the virgin, or original, material thickness throughout the walls of the vessel. Any reduction or thickening of material occurs by accident, or within the basic tolerance of established drawing techniques, which effect a maximum reduction of less than ten percent of the material thickness, more typically in the range of five to six percent. Variation in material thickness may also occur due to clearances within the dies employed in the drawing process. In some cases, die clearances are designed to create localized stretching in the material. Stretching, during a drawing operation, does not, however, impart any improved or advantageous properties to the stretched portion of the drawn metal product.
Nevertheless, drawing is an established and extensively used technique for the formation of pressure vessels, since it does permit formation of a pressure vessel of, typically, cylindrical configuration and having a closed end, without the use of seams. For example, earlier techniques would employ a flat sheet rolled into a cylinder, requiring the formation of a side seam, with the further addition of an end piece suitably seamed to the open end of the cylindrical structure. Drawing thus eliminates the side seam and the end seam of such earlier prior art pressure vessels. While affording these advantages, drawing imposes limitations as discussed above, and moreover fails to provide the capability of precisely sizing the vessel, not only as to its principal dimensions and particular diameter, but also, and to some extent more critically, as to the cross-sectional thickness of the walls of the metal vessel; in drawing operations, the latter is solely dependent upon the original or virgin material thickness. In view of the inability of drawing processes to size or control the material cross-sectional thickness, it is necessary, in the fabrication of drawn pressure vessels, to select a virgin material having a minimum thickness in light of its thickness tolerance, which is of sufficient strength to meet the maximum requirements of a pressure vessel, and to employ that same thickness throughout the entire vessel structure.
There is known in the art a fabrication technique termed "ironing", whereby metal is physically thinned by surface extension. U.S. Pat. No. 3,733,881 issued May 22, 1973 to Donald C. Grigorenko discloses a method and apparatus for making deep drawn and ironed metal shells wherein a metal disc is subjected to both drawing and ironing operations. In the particular process disclosed in that patent, a flat metal blank of virgin material is subjected to a reverse draw operation whereby the blank is shaped into a cup, or shell, of a first diameter and then is redrawn in an opposite direction to form a narrower diameter cup, or shell, of elongated axial length. The elongated shell is then subjected to successive ironing stages to reduce the side-wall thickness and thus elongate the axial dimension of the shell. The ironing technique is known to improve the physical characteristics of the ironed material in the side-walls. The conventional drawing and ironing process, however, has been used heretofore primarily for realizing material reduction, i.e., reducing the amount of material required to form a vessel of a given size, along with the elimination of the side and end seams as heretofore achieved by drawing operations alone.
Prior art drawing and ironing techniques, however, have not been suitable for the production of pressure vessels, such as oil filters for automotive applications, since such vessels require structural characteristics not heretofore capable of being achieved in shells fabricated by known drawing and ironing techniques.